1. Field of the Invention
The invention relates to an electric power steering system that assists a vehicle steering operation.
2. Description of the Related Art
There is an electric power steering system that assists a driver's steering operation by applying assist force generated by a motor to a steering mechanism of a vehicle. A conventional electric power steering system of this type is described in Japanese Patent Application Publication No. 2011-203091 (JP 2011-203091 A). The electric power steering system includes a torque sensor and a controller that controls driving of a motor. The torque sensor includes a Hall IC and a magnetic circuit that applies magnetic flux corresponding to steering torque applied by a driver to the Hall IC. In the electric power steering system, when the steering torque varies in response to a driver's steering operation, the magnetic flux applied from the magnetic circuit to the Hall IC of the torque sensor varies. Thus, a detection signal corresponding to the steering torque is output from the torque sensor. The controller computes the steering torque on the basis of the detection signal from the torque sensor, and computes an assist command value on the basis of the detected steering torque value. The controller applies assist force to the steering mechanism by controlling driving of the motor so as to cause the torque output from the motor to follow the assist command value.
The electric power steering system described in JP 2011-203091 A includes a magnetic field generator that periodically applies a magnetic field to the torque sensor, aside from the magnetic circuit. When a magnetic field is applied to the torque sensor through the use of the magnetic field generator, the controller determines whether a signal based on the applied magnetic field is output from the torque sensor. When the signal based on the applied magnetic field is not output from the torque sensor, the controller determines that a malfunction has occurred in the torque sensor. The controller detects the steering torque on the basis of the detection signal from the torque sensor immediately before the magnetic field is applied to the torque sensor from the magnetic field generator, and the controller holds the detected steering torque value during a period in which the magnetic field is generated from the magnetic field generator.
As described in, for example, Japanese Patent Application Publication No. 2006-131191 (JP 2006-131191 A), there is an electric power steering system that computes a first-order time differential value of a detected steering torque value on the basis of the detected steering torque value and provides compensation to an assist command value on the basis of the computed torque differential value.
In the electric power steering system described in JP 2011-203091 A, the detected steering torque value is constant during a period in which the detected steering torque value is held. When the controller computes the torque differential value in the period, the torque differential value is zero and the state where the torque differential value is zero continues. When the torque differential value is fixed to zero in this way, it is not possible to appropriately make compensation based on the torque differential value. In this regard, there is still room for improvement.